Vogel ?

Hetalia © by Hidekaz Himaruya

Oh Prussia, you're just...

"Du hast wohl einen Vogel (鳥)" is a german proverb (Sprichwort (ことわざ)), taken as a serious insult and saying "You are stupid (馬鹿)". It is actually literally translated as "You surely have a bird".

Prussia: "Genau wie ich! Großartig!" --> "Just like me! Awesome!"

So, yes, the joke is pretty lame... XD